linux-kernel - Re: [RFC PATCH 0/7] runtime format string checking

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Date:   Thu, 1 Nov 2018 23:06:23 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@...r.kernel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        linux-hwmon@...r.kernel.org,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [RFC PATCH 0/7] runtime format string checking

On 2018-10-30 21:58, Kees Cook wrote:
> On Sat, Oct 27, 2018 at 12:24 AM, Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=ce9b938574042d09920650cb3c63ec29658edc87
> The above seemed to "noisy" to send, but perhaps we should just land
> it anyway. They really _should_ be const.
>

Isn't that 063246641d4a9e9de84a2466fbad50112faf88dc in mainline ;) ?
BTW, I don't agree with all the changes in there: For auto variables, this

-       const char *cur_drv, *drv = "acpi-cpufreq";
+       const char drv[] = "acpi-cpufreq";
+       const char *cur_drv;

makes gcc actually generate that string on the stack instead of just
referring to an anonymous object in .rodata; one gets code gen like

+:      31 c0                   xor    %eax,%eax
+:      48 b8 61 63 70 69 2d    movabs $0x7570632d69706361,%rax # "acpi-cpu"
+:      63 70 75
+:      c7 44 24 0b 66 72 65    movl   $0x71657266,0xb(%rsp) # "freq"
+:      71
+:      c6 44 24 0f 00          movb   $0x0,0xf(%rsp) "\0"
+:      48 89 44 24 03          mov    %rax,0x3(%rsp)

It's not the-end-of-the-world-horrible, but it's better avoided,
especially for patches that are not supposed to change anything. And
longer strings would of course produce even more gunk like the above.
A better fix which also silences -Wformat-security is to declare the
variable itself const, i.e.

const char *const drv = "acpi-cpufreq".

Yes, gcc should be able to infer the constness of drv from the fact that
it's never assigned to elsewhere in the function... I think I saw that
on some gcc todo list at some point.

> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=b7dcfc8f48caaafcc423e5793f7ef61b9bb5c458
> This one covers cases where the pointer is pointing to a const string,
> so really there's no sense in injecting the "%s", but I was collecting
> them to make real ones stand out.

I don't agree. Yes, a human can verify that _currently_, only "pencrypt"
and "pdecrypt" can ever reach pcrypt_sysfs_add(). But without the
compiler being smart enough to do that, one will never know if some new
caller shows up, or one of those literals grows a % for some reason.
Adding "%s" doesn't cost much, especially not in cases (like this one)
where the fmt+args end up at kobject_set_name_vargs - for a "%s" +
literal that does a (succesful) kstrdup_const(), so we never even hit
the vsnprintf() engine.

>> Patches 5,6,7 are
>> some examples of where one might add fmtcheck() calls. I don't think
>> we can get to a state where we can unconditionally add
>> -Wformat-nonliteral to the build, but I think there's a lot of
>> low-hanging fruit.
> 
> How much work do you think it'd take to get to a
> format-nonliteral-clean build? I think it's worth doing the work if
> it's not totally intractable.

Probably less than the VLA removal. But it kind of depends on which
tools one allows. I can't see how to do it without something like
fmtcheck() to annotate certain places (e.g. the nfs example). Maybe a
no_fmtcheck() to annotate places which have been manually verified
[modulo the above "but that may change..."] would also be needed
(no_fmtcheck would be the same as fmtcheck for at !CONFIG_FMTCHECK
kernel), similar to how we have no_printk.

I kind of agree with Guenther that the hwmon example is a bad one. It
would be better to have the compiler check all those string literals
against a pattern at build time. Probably the format template plugin can
be extended to apply to any "const char*" declaration, not just those
sitting inside structs. But I'd rather get fmtcheck() in first before
returning to work on that plugin.

Rasmus